99 Yukon-Engine hunts on slight grades

[FTGF]

:confused2:A little Background- Last week while coming back from the beach the engine lost power and RPM's dropped while driving up a hill at highway speed. This happened twice. Pulled over checked trans fluid and it was good. Called local $$$ Dealer and they suggest Catalytic converter. Was quite skeptical since I have never had one fail (that didn't rust!)

Brought it in and they tested fuel pump pressure and Cats and said the Cats were clogged. They wanted almost $1k for new ones. Bought Direct bolt Cat replacements at local AutoZ. Unfortunately truck stalled and would not start again on way to Midas, had to be towed.

Midas installed for ~$100, but truck still wouldn't start. Midas tech found fuel pump went bad when the Cats clogged. (I guess that's possible since there is so much back pressure???) I didn't hear the normal fuel pump hum so I agreed, and since they had it and wouldn't let me bring anymore of my own purchased parts in they replaced it.

They suggested I also change all 4 oxygen sensors since they may have caused the cats to fail. They wanted over 200 a piece for them but the local AutoZ sold them all (Bosch) for ~260.

Now my issue is when I am driving up slight grades the engine seems to hunt a little. The RPM's are below 2k and you can't see any movement in the Tach but you feel it (and hear it) in the pull of the truck. If you step on the gas it down shifts and takes right off. Coasting no issues. The truck runs well accelerating and at highways speeds and level roads.

Should the muffler be replaced as well when you do the cats? Any ideas?
I have 112k miles on it and I do tow a tandem axle trailer from time to time but have never pulled more than 3.6k lbs. Mostly short haul type stuff.

Any suggestions would be much appreciated

 

[GMCYukalade]

A full tune-up... and also - you should change the fuel filter, especially since you changed the pump - use AC-Delco plugs wires etc - and keep an eye on those O2 sensors as well, Bosch is no where near as good as the factory ones.. - if you didn't have a service engine soon light on than you probably didn't need o2s and you probably def. didn't need rear-o2s - but you never know - the muffler shouldn't clog at all unless you stuffed some t-shirts in it..

 

[FTGF]

I did the inline fuel filter back in November. I did Plugs, cap, rotor, PCV and wire in November also. I did the Air filter on Sunday and an oil and filter in the beginning of June. Am I missing anything else in the tune up?

When the gas tank was dropped it was clean and the old pump sock (ok it catches the big stuff) was spotless.

O2's were based on feedback from guys at Midas. I got an 8yr/80k mile warranty on cats and wanted to make sure it could not be questioned by bad sensors. Midas says when there cats fail early and they send them back the mfg always says the vehicle had

I had an SES light on a couple of weeks ago but that was a code 442 and it was the EVAP canister vent valve.

No t-shirts in the exhaust, I just didn't know how prone they are to clogging since the cats were clogged. As far as I can tell the muffler is original.

Any other ideas, should I go ahead and change the fuel filter again even though I just did it seven months ago?

 

[GMCYukalade]

Well, what you describe to me.. is that it is missing at a very low RPM while pulling hills.. correct? - What kind of plugs did you use when you did the tune-up? if your cats clogged, and the fuel pump went bad.. i wonder what it could have done to the plugs too?

 

[98hoe]

Could have fouled a plug when the cats failed. Witch one will be hard to determine since it does not show symptoms at idle. Never hurts to put new plugs in. That's what I would do.

 

[FTGF]

You raise an excellent point. I guess I just didn't think of it that way. I would have thought you would see a missing issue in other scenario's not just under a load condition.

I put Autolite plugs in the last time. I will buy new ones (plugs, cap, rotor and wires) and put them in as soon as I can. I will change the inline fuel filter also since they are cheap and it takes another piece out of the puzzle.

Is there anything else I should try? The fewer trips to the AutoZ with gas at 4.00 a gallon the better...

Thank you for the help.

 

[98hoe]

I would just try plugs. I don't think you need to replace all that. Especially since you just did 7 months ago. I mean it wont hurt, just might be a waste.

 

[96ProCompTahoe]

you may want to take the cap off and look at it and the rotor, if they look good theres no point in replacing them. the cat. shouldn't have affected the wires any either.

i would have to put my money on the plugs.

 

[GMCYukalade]

yea i second everyone else - you really should not replace the wires or anything else - just go with ac delco plugs... - and plus the wires and everything else should still be under a warranty

 

[Whippled2dr]

What speed are you traveling on the slight grade and how my throttle are you giving it (percentage)? It could be your torque converter unlocking and locking which is normal but you would probably notice the rpms fluctuate a little.

Another thing that you could check just in case would be your distributor gear. For a while I was getting a random misfire during cruise (you could feel it) that I just couldn't figure out. At first I thought it was my fuel pump dieing or some fuel related issue. It ran pretty good at WOT (AFR was doing what I wanted it to do) and fuel pressure was great so I didn't mess with anything. I then thought it was spark related. I swapped out to another coil I had and it made no difference. New cap, rotor, wires and plugs and again still got my slight misfire I couldn't get rid of. So then I decided to check my distributor gear for any wear and I found my culprit. GM part# 10457356 for the replacement and it fixed my problem.

Just throwing out some ideas.

 

[FTGF]

Thanks for all the advice!

Just finished putting some AC Delco RapidFire platinum plugs in tonight. It looked to me like two of them were in worse shape than all the others. Will see how it does on the drive to work tomorrow.

 

[GMCYukalade]

Hope it fixes it - but honestly you didn't need the rapid fire platinum stuff - all you need is regular AC Delco plugs.. nothing fancy.. - fancy plugs have a tendancy to make our motors not act right.

 

[FTGF]

Alright, no joy there. I will continue with the rest of a normal TU and hope that gets it.

I will go after the distributor last since I haven't pulled one in a very, very long time......

Thanks again to all of you for the advice and pointers, it's much appreciated. I just hate going to the $tealer for service.....

 

[GMCYukalade]

It could really be the plugs this time... - remember our tahoes/yukons don't like fancy plugs - they burn the gas to quick, and still makes it miss/shudder.. - AC Delco Factory replacements are fine..